Generally, as osteosynthesis means for treating fractures of upper portions of femurs, intramedullary nails comprising a nail body to be intramedullarily inserted from proximal parts of femurs have been used. These intramedullary nails include, for example, one as described in the specification of European Patent Publication NO. 0257118. This has an axial hole extending from an opening provided at a proximal end portion toward a distal end portion in an axis direction and a transverse hole formed in a direction intersecting with the axis direction, and by inserting a bone fastener (bone screw) through this transverse hole and inserting and fixing its distal end inside the bone, a bone fracture is prevented or a rotation of the intramedullary nail is prevented.
In this type of intramedullary nail, by screwing an adjusting screw (set screw) into the axial hole through the opening provided in its proximal end portion, the distal end of this adjusting screw is engaged with the outer circumferential surface of the bone fastener inserted through the transverse hole, whereby the bone fastener is prevented from rotating with respect to the nail body or is completely fixed.
On the other hand, when the above-described intramedullary nail is used in a case where a fracture line exists between the femoral bone head portion and tubular portion, there is a danger that at the end of the femur, the bone head portion may be rotated around the axis line of the bone fastener inserted in the bone head portion. For prevention thereof, as described in, for example, U.S. Pat. No. 5,531,748, in some cases, a plurality of transverse holes are provided in the nail body, and a plurality of bone fasteners respectively inserted through these transverse holes are inserted into the bone head portion.
However, in the aforementioned intramedullary nail, although it is necessary to screw the adjusting screw into the axial hole of the nail body and engage its distal end with the bone fastener after inserting the bone fastener through the transverse hole of the nail body and screwing the same toward the inside of the femur, since the engaged condition with the bone fastener is adjusted by a screwing amount of the adjusting screw, the degree of adjustment is difficult, therein a problem has existed.
In greater detail, in most cases, on the outer circumferential surface of the bone fastener, an engaging groove extending in its axis direction has been formed. And, by engaging the distal end of the adjusting screw with this engaging groove, the bone fastener can be brought into a condition (slide-free condition) where the bone fastener is, with respect to the nail body, slidable in its axis direction but cannot rotate around its axis line, and in addition, by strongly bringing the distal end of the adjusting screw into contact against the bottom surface of the engaging groove, the bone fastener can also be brought into a condition (slide-lock condition) where the bone fastener has been completely (including in its axis direction) fixed to the nail body.
Nevertheless, generally, since the bone fastener is often provided in the slide-free condition with respect to the nail body, it is necessary to subtly adjust the screwing amount of the adjusting screw in order to obtain this slide-free condition. For example, on a surgical site, normally, subtle operations such as returning the adjusting screw by a predetermined amount from a position where the adjusting screw has been completely screwed so that the bone fastener reaches the slide-lock condition have been performed, and it is impossible to completely deny a risk of the fastener reaching the slide-lock condition or the fastener reaching a perfectly free condition (being rotatable around the axis line thereof) to the contrary if this delicate operation is mishandled. In addition, there is also a problem such that, since the adjusting screw has been merely screwed into the nail body, the adjusting screw is not sometimes correctly determined owing to a looseness of this screwed structure, this encourages failure of the operation, and operation resistance increases if the looseness of the screwed structure is reduced, therefore the operation becomes difficult.
Furthermore, in the case where a plurality of transverse holes are formed in a nail body and two or more bone fasteners are inserted (screwed) into a femoral bone head portion, there is a problem such that, bone fasteners inserted through transverse holes formed on the proximal end side of the nail body can be engaged and retained by the adjusting screw inserted through an opening provided at a proximal end portion of the nail body, whereas for bone fasteners inserted through the side of a distal end portion, the adjusting screw cannot be engaged therewith by being obstructed by the bone fasteners inserted through the proximal end portion side.